Problem with `Expect: 100-continue` handling with isahc

the isahc client seems to hang indefinitely when speaking to an async-h1 based server when there is a request body

Start by reproducing a request with a body using isahc against an async-h1 based server, then trace the Expect: 100-continue handling in the client and server interaction. Done means the request no longer hangs indefinitely and the behavior is covered by a regression test.

Written by the indexing model from the issue text.